A wide range of consumer, commercial, and industrial applications utilize patch antennas in GNSS applications which can determine locations with high accuracy. Currently deployed systems include the United States Global Positioning System (GPS) and the Russian GLONASS, and others such as the European GALILEO system are under development.
In a GNSS, a navigation receiver receives and processes radio signals transmitted by satellites located within a line-of-sight of the navigation receiver. A critical component of a GNSS is the receiver antenna. Key properties of the receiver antenna include bandwidth, multipath rejection, size, and weight. High-accuracy navigation receivers typically process signals from two frequency bands. For example, two common frequency bands are a low-frequency (LF) band in the range of 1164-1300 MHz, and a high-frequency (HF) band in the range of 1525-1610 MHz.
One reason for reduced GNSS positioning accuracy of land objects is related to receiving not only line-of-sight satellite signals but also signals reflected from surrounding objects, and especially from the Earth's surface (i.e., the ground). The strength of such signals depends directly on the antenna's directional pattern (DP) in the rear hemisphere. A right-hand circularly polarized signal is used as a working signal in navigation systems. As will be appreciated, a low level of directional pattern in the lower hemisphere (particularly in the nadir direction) is a standard antenna requirement, and typically a reduction in the antenna's weight and overall dimensions is desirable.
It is well-known that patch antennas are widely used in GNSS applications due to certain technical and operational advantages such as low height which enables low-profile patch antennas to be constructed. As will be understood, a conventional patch antenna typically includes a radiating patch located over a ground plane such that the lateral dimension (i.e., length) of the ground plane is longer than that of the patch. To provide qualitative signal reception from navigation satellites across the celestial hemisphere up to angles close to the horizon, the patch antenna should also have a wide enough Directional Pattern (DP) in the forward (i.e., upper) hemisphere. The width of a patch antenna DP is determined by the length of the patch such that the shorter the patch is, the wider the DP will be. The length of the patch is normally 0.2 . . . 0.3λ, wherein λ is the wavelength in free space and the minimal length is determined by the operational bandwidth. To provide for a resonance mode on such lengths, a dielectric between the ground plane and patch or capacitive elements is used.
A considerable contribution to positioning errors in GNSS systems is attributable to signal(s) reflected from the ground. To reduce this multipath error, a low DP level should be provided in the backward hemisphere, and one conventional solution is to choose a ground plane length equal to at least 0.5λ. The size of the ground plane determinates the overall antenna dimension, and the aforementioned wavelength corresponding to the minimal frequency of the operation range. For GNSS, this frequency is 1164 MHz, which corresponds to 258 mm which translates to an antenna size of at least 130 mm. Any further reduction in the length of the ground plane results in a noticeable increase in DP level in the backward hemisphere. If the length of the ground plane is equal to that of the patch, the DP level in the backward hemisphere is the same as in the forward hemisphere which is unacceptable for the standard operation of high-precision GNSS receivers. Therefore, a minimal dimension of standard patch antennas is limited by the length of the ground plane which provides the desired low level of DP in the lower hemisphere, and particularly in the nadir direction (i.e., the desired level of multipath suppression).
One example of an antenna providing for low DP level in the nadir direction is described in U.S. Pat. No. 9,184,503 where the antenna's design includes a length of ground plane that is equal to or smaller than the length of the patch. To achieve this design, a loop radiator is located around the patch whereby the radiator is excited by dual-wire lines connected to a separate power supply. The power supply provides excitation of the loop radiator with such amplitude and phase that the field of the patch is subtracted from the field of the loop radiator. However, potential drawbacks of such a design are the overall design complexity and the requirement of a separate supply line to power the loop radiator.
Therefore, a need exists for an improved high-precision GNSS antenna design with lower complexity, smaller dimensions, and efficient multipath suppression.